1. Field of the Invention
The present invention relates to an active matrix substrate incorporated, for example, in a liquid crystal display device.
2. Description of the Related Art
An active matrix type liquid crystal display device is known which is provided with a so-called active matrix substrate. Such an active matrix substrate includes thin film transistors (hereinafter, simply referred to as "TFTs") having channel regions made of, for example, amorphous Si, or metal insulator metal elements (i.e., MIM elements) on a substrate along with a plurality of gate lines and data lines, the gate lines and the data lines intersecting each other.
Conventionally, an inorganic material such as silicon nitride (SiN) is used as an insulating film for the active matrix substrate. Recently, an organic material such as a photosensitive transparent acrylic resin is also used as the insulating film.
As described, for example, in Japanese Laid-Open Patent Publication No. 58-172685, the insulating film made of the organic material, i.e., an organic insulating film is expected to be widely used as an interlayer insulating film utilized in a high aperture ratio structure of a liquid crystal display device.
FIG. 2A is a plan view showing a one-pixel portion of an active matrix substrate in a liquid crystal display device having a high aperture ratio structure by using an organic insulating film. FIG. 2B is a cross-sectional view showing the same active matrix substrate taken along line II--II of FIG. 2A. Referring to FIGS. 2A and 2B, gate lines 1 having protruding portions and common lines 3 are provided on an insulating substrate 9. The common lines 3 extend in parallel with the gate lines 1. A gate insulating film 8 is provided on the gate lines 1 and the common lines 3 so as to cover the insulating substrate 9, on which data lines 2 having protruding portions and drain electrodes 4 are provided. Each of the protruding portions of the gate lines 1 acts as a gate electrode and each of the protruding portions of the data lines 2 acts as a source electrode. The gate electrode, the source electrode and the drain electrode 4 form each of TFTs 7, i.e., switching elements. An organic insulating film 10 is provided on such a structure so as to cover the entire surface of the substrate 9. Pixel electrodes 6 are provided on the organic insulating film 10 so as to make contact with the respective drain electrodes 4 via respective contact holes 5 formed through the organic insulating film 10. By laying the pixel electrodes 6 over the gate lines 1 and the data lines 2 in the above-described manner, an active matrix substrate with an increased aperture ratio can be obtained.
As can be appreciated from the above, in order to form the active matrix substrate having the increased aperture ratio structure shown in FIGS. 2A and 2B, a step of forming a transparent conductive film (i.e., the pixel electrode 6) on the organic insulating film 10 is essential. However, heat resistance of the organic insulating film is inferior compared to that of an inorganic insulating film. Therefore, methods disclosed in Japanese Laid-Open Patent Publication No. 5-346575 and Japanese Laid-Open Patent Publication No. 6-88973 may be used for forming a transparent conductive film which has equivalent characteristics (e.g., in terms of accurate patterning upon etching, low electric resistance and uniformity) as those of a transparent conductive film formed on an inorganic insulating film.
In the methods disclosed in Japanese Laid-Open Patent Publication No. 5-346575 and Japanese Laid-Open Patent Publication No. 6-88973, the transparent conductive film is formed on the organic insulating film as follows. A transparent conductive film is deposited on an organic insulating film at a sufficiently low temperature, i.e., approximately room temperature. Then, the obtained film is annealed in an atmosphere such as air or hydrogen which is different from the atmosphere used to form the transparent conductive film and at a temperature higher than the temperature employed to deposit the film but equal to or lower than a temperature at which the organic insulating film can withstand the heat.
However, in the case where the annealing is conducted in a film formation apparatus by the above-described method, time for heating and cooling the insulating substrate, and time for exchanging gas is required. Thus, the processing performance of the film formation apparatus is significantly deteriorated. As a result, an annealing apparatus needs to be provided separately from the film formation apparatus, in which case the number of the production steps are increased.